Field of the Invention: The present invention relates in general to stacked integrated circuit (IC) dice, and, in particular, to devices and methods for stacking wire-bonded IC dice on flip-chip bonded IC dice.
State of the Art: Integrated circuit (IC) dice or “chips” are small, generally rectangular IC devices cut from a semiconductor wafer, such as a silicon wafer, on which multiple ICs have been fabricated. Traditionally, bare IC dice are packaged to protect them from corrosion by enclosing them in die packages. Such packages work well to protect IC dice, but they can be more bulky than desirable for certain multi-chip applications requiring compact die packaging.
Accordingly, a variety of compact die packaging techniques exists. In one such technique, the back-side surface of a bare IC die is directly mounted on the surface of a Printed Circuit Board (PCB), and bond pads on the front-side surface of the bare die are then wire-bonded to wire-bondable pads on the surface of the PCB to interconnect circuitry in the die with external circuitry through conductive traces on the PCB. This technique may be referred to as “Chip-On-Board (COB) with wire-bonding.” In another such technique, conductive “bumps” on the front-side surface of a bare IC die are bonded to “flip-chip” pads on the surface of a PCB to interconnect circuitry in the die with external circuitry. Both the COB with wire-bonding technique and the flip-chip technique are well known to those of skill in the field of this invention, and are described in more detail in U.S. Pat. Nos. 5,422,435, 5,495,398, 5,502,289, and 5,508,561.
While these traditional compact die packaging techniques are more compact than the bulky die packages described above, they still are not compact enough for some multi-chip applications requiring many chips in a small area. For example, an ever-growing demand for Dynamic Random Access Memory (DRAM) capacity is driving a need for ever-more DRAM memory chips to be packed into a small area.
As a consequence, a variety of techniques exists for stacking chips on top of one another to increase the number of chips provided in a small area. As described in U.S. Pat. Nos. 5,228,192, 5,252,857, and 5,514,907, some of these techniques involve mounting individual bare IC dice, or individual dice packaged using a traditional die package, to parts of a structure, and then assembling the structure so the mounted dice are stacked. Also, as described in U.S. Pat. No. 5,323,060, another technique involves stacking bare IC dice on top of one another, and then wire-bonding the dice to a PCB and to one another. Similarly, as described in U.S. Pat. No. 5,399,898, a further technique involves stacking exotic IC dice having conductive bumps or pads on both sides on top of one another and then flip-chip bonding the dice to a PCB and to one another. Further, as described in U.S. Pat. Nos. 5,422,435, 5,495,398, and 5,502,289, an additional technique involves stacking bare IC dice on top of a die mounted to a PCB using the COB with the wire-bonding technique described above. These stacked dice are then wire-bonded to the PCB and to one another, or are interconnected using flip-chip bumps, or both. In addition, as described in U.S. Pat. No. 5,527,740, a still further technique involves back-side bonding a pair of IC dice to one another and then wire-bonding bond pads on the front sides of the bonded dice to a PCB.
While all of these stacking techniques work well to increase the density of chips provided in a given area, they do not provide a simple stacking technique for IC dice flip-chip mounted to a PCB in the manner described above. They also do not provide a stacking technique that can be used to repair or replace a defective IC die flip-chip mounted to a PCB. Such “repair” stacking techniques are well known for IC dice mounted to a PCB using the COB with wire-bonding technique, as described in U.S. Pat. No. 4,567,643.
Therefore, there is a need in the art for a simple stacking technique for IC dice flip-chip mounted to a PCB or any other base, and the inventive technique should be useful for repairing or replacing flip-chip mounted IC dice.